1. Field of the Invention
The present invention relates to a magnetoresistance effect type head using a magnetoresistance effect film for reading the magnetic field intensity of a magnetic recording medium or the like as a signal, particularly a spin-valve type magnetoresistance effect film which is capable of reading a small magnetic field change as a greater electrical resistance change signal.
2. Description of the Prior Art
Recently, there has been the development for increasing the sensitivity of magnetic sensors and increasing the density in magnetic recording and, following this, magnetoresistance effect type magnetic sensors (hereinafter referred to as MR sensors) and magnetoresistance effect type magnetic heads (hereinafter referred to as MR heads) using magnetoresistance change have been actively developed. Both MR sensors and MR heads are designed to read out external magnetic field signals on the basis of the variation in resistance of a reading sensor portion formed of magnetic material. The MR sensors have an advantage that a high sensitivity can be obtained and the MR heads have an advantage that a high output can be obtained upon reading out signals magnetically recorded in high density because the reproduced output does not depend on the relative speed of the sensors or heads to the recording medium.
However, conventional MR sensors which are formed of magnetic materials such as Ni.sub.80 Fe.sub.20 (Permalloy), NiCo or the like have a small resistance change ratio .DELTA.R/R which is about 1 to 3% at maximum, and thus these materials have insufficient sensitivity as the reading MR head materials for ultrahigh density recording of the order of several GBPSI (Giga Bits Per Square Inches) or more.
Attention has been recently paid to artificial lattices having the structure in which thin films of metal having a thickness of an atomic diameter order are periodically stacked, because their behavior is different from that of bulk metal. One of such artificial lattices is a magnetic multilayered film having ferromagnetic metal thin films and non-magnetic metal thin films alternately deposited on a substrate. Heretofore, magnetic multilayered films of an iron-chromium type, a cobalt-copper type and the like have been known. However, these artificial lattice magnetic multilayered films are not commercially applicable as they are because the external magnetic field at which a maximum resistance change occurs (operating magnetic field intensity), is as high as several tens of kilo-oersted.
Under these circumstances, a new structure which is called a spin valve has been proposed. In this structure, two NiFe layers are formed via a non-magnetic metal layer, and an FeMn layer is further formed so as to be adjacent to one of the NiFe layers.
In this case, since the FeMn layer and the NiFe layer adjacent thereto are directly exchange-coupled to each other, the direction of the magnetic spin of this NiFe layer is fixed in the range of several tens to several hundreds Oe in magnetic field intensity. On the other hand, the direction of the magnetic spin of the other NiFe layer is freely changeable by an external magnetic field. As a result, there can be achieved a magnetoresistance change ratio (MR ratio) of 2 to 5% in a small magnetic field range which corresponds to the degree of coercive force of the NiFe layer.
In the spin valve, by realizing a difference in relative angles of spins between two magnetic layers, the large MR change which differs from the conventional anisotropy magnetoresistance (AMR) effect is accomplished. This is realized by pinning of the magnetic layer spin due to the direct exchange coupling force between one of the magnetic layers and the antiferromagnetic layer. This exchange coupling can be the substance of the spin valve.
However, for putting the spin valve to practical use, there are various problems as described hereinbelow. The strength of the exchange coupling pinning the magnetic layer is represented by a magnitude of a unidirectional anisotropic magnetic field Hua which shifts. On the other hand, a temperature at which Hua is lost is set to be a blocking temperature Tb which represents a thermal stability. The generally used FeMn layers and other antiferromagnetic exchange coupling:
(1) The blocking temperature Tb is low, that is, in the range from 150 to 170.degree. C. As compared with the state of the bulk, the blocking temperature Tb is low so that an excellent thin film which can fully achieve an expected pinning effect can not be obtained.
(2) Dispersion of the blocking temperatures Tb occurs. Specifically, because of a thin film, the film surface of the FeMn layer is composed of various crystal grains, and the individual crystal grains have their own blocking temperatures Tb. If all the crystal grains have the same blocking temperature Tb, no problem is raised. However, actually, some crystal grains have lower blocking temperatures Tb, while some crystal grains have higher blocking temperatures Tb. As a result, it is possible that there exist those grains with small exchange coupling which causes reversal of the spin in the ferromagnetic layer at 80 to 120.degree. C. representing an operating temperature range on application (due to existence of crystal grains having lower blocking temperatures Tb). Then, the spin direction of the pinned ferromagnetic layer may be inclined as a whole so that the output voltage is lowered. Thus, it is desired that a high-quality antiferromagnetic thin film be provided wherein as many crystal grains as possible have the same high blocking temperature Tb.
For solving the foregoing problems, antiferromagnetic thin films made of Ru.sub.x M.sub.y Mn.sub.z (M represents at least one selected from Rh, Pt, Pd, Au, Ag and Re) have been proposed as preferred examples in Japanese Patent Applications Nos. 8-357608 and 9-219121.
On the other hand, for causing a magnetoresistance effect film to function as a spin valve film, it is necessary to orthogonalize (ideally at 90 degrees) in advance magnetization directions of a soft magnetic layer and a ferromagnetic layer forming the magnetoresistance effect film. This orthogonalization process can be carried out by applying mutually orthogonal magnetic fields upon the formation of the soft magnetic layer and the ferromagnetic layer. However, due to an unavoidable heating step (for example, resist curing at 250.degree. C. for 3 hours in total) in the manufacturing process after the formation of the film, the orthogonalization of magnetization is disturbed to largely deviate from ideal 90 degrees so that the MR ratio is lowered. As a result, there is raised a disadvantage that the sufficient output can not be achieved after assembled as a spin valve head. For this reason, it is desired that the orthogonalization process of magnetization directions be carried out at the end of the head manufacturing process. Specifically, it is necessary to heat the head, approximate to a final product, to no lower than a blocking temperature Tb and apply again magnetic fields for adjusting the spin directions of the magnetoresistance effect film. However, as described above, an antiferromagnetic layer having a high blocking temperature Tb is used for ensuring the high pinning effect. Thus, a heat treatment temperature for the orthogonalization is quite high, i.e. about 300.degree. C., and required over a long time. The orthogonalization process at such a high temperature may cause mutual diffusion of substances between the thin film magnetic layers and lose an anisotropic magnetic field Hk of the soft magnetic layer so as to deteriorate a magnetic characteristic, thereby leading to a damage to the head. As a result, when an antiferromagnetic layer having a high blocking temperature Tb is used, the complete orthogonalization of magnetization of a soft magnetic layer and a ferromagnetic layer can not be realized so that the sufficient output of a spin valve head can not be obtained.